I think Gabe is definitely capable of beating Lesnar.
Gonzaga has better striking, and should take the fight if they stay on their feet. I think when they're on the feet Lesnar needs to really push the pace, throw hard, and use his strength to push Gabe around.
Brock Lesnar has a size and strength advantage, but Gonzaga is a pretty big guy, big enough to neutralize this to some degree, though not entirely.
If Lesnar tries to take the fight to the ground to bully Gonzaga, obviously he needs to be VERY careful of Gabriel's submission's. If he isn't, eventually he will get caught.
It all depends on what Lesnar evaluates as the biggest threat, and how he thinks his skills match up with Gonzaga's. Whether he feels that Gabe's submissions are the biggest risk, or his strikes are.
If he tries to stand with Gonzaga, and doesn't like what he experiences, as a wrestler, taking the fight to the ground will be instinctual.
An interesting fight if it were to come about. We'll see how Lesnar fairs against Couture.